44 research outputs found
Backreaction effects of dissipation in neutrino decoupling
Dissipative effects during neutrino decoupling in the early universe create a small backreaction on the Hubble rate, and lead to a small rise in temperature and entropy. We use a simplified thermo-hydrodynamic model, which provides a causal approximation to kinetic theory, in order to estimate the backreaction effects and the entropy production
Density perturbations with relativistic thermodynamics
We investigate cosmological density perturbations in a covariant and gauge- invariant formalism, incorporating relativistic causal thermodynamics to give a self-consistent description. The gradient of density inhomogeneities splits covariantly into a scalar part, a rotational vector part that is determined by the vorticity, and a tensor part that describes the shape. We give the evolution equations for these parts in the general dissipative case. Causal thermodynamics gives evolution equations for viswcous stress and heat flux, which are coupled to the density perturbation equation and to the entropy and temperature perturbation equations. We give the full coupled system in the general dissipative case, and simplify the system in certain cases
Inflation driven by causal heat flux
We find a simple inflationary solution in an inhomogeneous spacetime with
heat flux. The heat flux obeys a causal transport equation, and counteracts the
inflationary decrease of energy density. At late times, the heat flux tends to
zero and the fluid approaches the equation of state .Comment: Latex 5 pages; to appear Gen. Rel. Gra
Collisional equilibrium, particle production and the inflationary universe
Particle production processes in the expanding universe are described within
a simple kinetic model. The equilibrium conditions for a Maxwell-Boltzmann gas
with variable particle number are investigated. We find that radiation and
nonrelativistic matter may be in equilibrium at the same temperature provided
the matter particles are created at a rate that is half the expansion rate.
Using the fact that the creation of particles is dynamically equivalent to a
nonvanishing bulk pressure we calculate the backreaction of this process on the
cosmological dynamics. It turns out that the `adiabatic' creation of massive
particles with an equilibrium distribution for the latter necessarily implies
power-law inflation. Exponential inflation in this context is shown to become
inconsistent with the second law of thermodynamics after a time interval of the
order of the Hubble time.Comment: 19 pages, latex, no figures, to appear in Phys.Rev.
Quantum atom-light interfaces in the gaussian description for spin-1 systems
We extend the covariance-matrix description of atom--light quantum
interfaces, originally developed for real and effective spin-1/2 atoms, to
include "spin alignment" degrees of freedom. This allows accurate modeling of
optically-probed spin-1 ensembles in arbitrary magnetic fields. We also include
technical noise terms that are very common in experimental situations. These
include magnetic field noise, variable atom number and the effect of magnetic
field inhomogeneities. We demonstrate the validity of our extended model by
comparing numerical simulations to a free--induction decay (FID) measurement of
polarized Rb atoms in the ground state. We qualitatively and
quantitatively reproduce experimental results with all free parameters of the
simulations fixed. The model can be easily extended to larger spin systems, and
adapted to more complicated experimental situations.Comment: 25 pages, 4 figure
On the dual interpretation of zero-curvature Friedmann-Robertson-Walker models
Two possible interpretations of FRW cosmologies (perfect fluid or dissipative
fluid)are considered as consecutive phases of the system. Necessary conditions
are found, for the transition from perfect fluid to dissipative regime to
occur, bringing out the conspicuous role played by a particular state of the
system (the ''critical point '').Comment: 13 pages Latex, to appear in Class.Quantum Gra
Safety of the Intended Functionality Concept Integration into a Validation Tool Suite
Nowadays, the increasing complexity of Advanced Driver Assistance Systems
(ADAS) and Automated Driving (AD) means that the industry must move towards a
scenario-based approach to validation rather than relying on established
technology-based methods. This new focus also requires the validation process
to take into account Safety of the Intended Functionality (SOTIF), as many
scenarios may trigger hazardous vehicle behaviour. Thus, this work demonstrates
how the integration of the SOTIF process within an existing validation tool
suite can be achieved. The necessary adaptations are explained with
accompanying examples to aid comprehension of the approach
Stress effects in structure formation
Residual velocity dispersion in cold dark matter induces stresses which lead
to effects that are absent in the idealized dust model. A previous Newtonian
analysis showed how this approach can provide a theoretical foundation for the
phenomenological adhesion model. We develop a relativistic kinetic theory
generalization which also incorporates the anisotropic velocity dispersion that
will typically be present. In addition to density perturbations, we consider
the rotational and shape distortion properties of clustering. These quantities
together characterize the linear development of density inhomogeneity, and we
find exact solutions for their evolution. As expected, the corrections are
small and arise only in the decaying modes, but their effect is interesting.
One of the modes for density perturbations decays less rapidly than the
standard decaying mode. The new rotational mode generates precession of the
axis of rotation. The new shape modes produce additional distortion that
remains frozen in during the subsequent (linear) evolution, despite the rapid
decay of the terms that caused it.Comment: significantly improved discussion of kinetic theory of CDM velocity
dispersion; to appear Phys. Rev.